Osteopontin-hydroxyapatite interactions in vitro: inhibition of hydroxyapatite formation and growth in a gelatin-gel

Osteopontin is a phosphorylated bone matrix sialoprotein, postulated to play a regulatory role in biomineralization. The effects of a crude preparation of rat bone osteopontin and a more highly purified bovine bone osteopontin were evaluated using a gel diffusion system to measure effects of 0.1-100 micrograms/ml of this matrix protein on hydroxyapatite formation and crystal proliferation. Bovine osteopontin at concentrations greater than 25 micrograms/ml inhibited both hydroxyapatite formation and growth in a dose-dependent manner. Osteopontin at concentrations lower than 25 micrograms/ml had no detectable effect on the amount of mineral accumulated in experiments with and without pre-formed hydroxyapatite seed crystals either when initial mineral deposition was assessed at 3.5 days, or when mineral formation and growth were assessed at 5 days. There was a statistically significant dose-dependent decrease in crystal length at all concentrations tested. The rat osteopontin preparation had similar inhibitory abilities. Partial dephosphorylation of bovine osteopontin with alkaline phosphatase removed its inhibitory ability, and reduced its ability to bind calcium. The affinity of bovine osteopontin for hydroxyapatite was determined based on a Langmuir adsorption isotherm, with values of K (binding affinity) and N (number of binding sites) being 0.026 ml/microgram and 1084 micrograms/m2, respectively. The data suggest that, in this system, osteopontin is an effective inhibitor of hydroxyapatite formation and growth due to its affinity for the hydroxyapatite crystals. In this system, osteopontin, distinct from other phosphoproteins which both promote and inhibit hydroxyapatite deposition, did not enhance mineral formation at any concentration tested.

p-Bromophenacyl bromide potentiates hydrogen peroxide formation in human platelets challenged by thrombin and inhibits aggregation

p-Bromophenacyl bromide, a potent alkylating agent inhibitor of phospholipase A2 and adenylate cyclase, potentiates hydrogen peroxide production in human platelets activated with thrombin. The maximal cooperative effect, specific for thrombin as inducer and potentiated by exogenous calcium, was observed at 2 microM BPB. Since it was shown that BPB is also a strong inhibitor of platelet aggregation (IC 50 = 3 +/- 1 microM) it is likely to suppose that the abnormal accumulation of hydrogen peroxide can influence platelet function.

Cytoskeleton protein composition upon platelet stimulation with thrombin in essential thrombocythemia

BACKGROUND

Structural and functional platelet alterations are present in essential thrombocythemia (E.T.). Platelet aggregation patterns are highly variable; moreover, structural, biochemical and metabolic platelet defects have also been shown. In this study, data on function and structure of platelets from a group of patients affected with E.T. are reported. Since cytoskeleton proteins are involved in activation and aggregation mechanisms, we investigated the cytoskeleton protein composition of resting, activated and aggregated platelets using electrophoretic analysis.

PATIENTS AND METHODS

Studies were performed on 13 patients affected with E.T. On the basis of their functional behavior in response to thrombin, patients were divided into two groups: T1 (5 patients) and T2 (8 patients). A group of 11 healthy subjects was examined as control. Washed platelets were stimulated with 0.05 or 0.25 U/mL thrombin in the presence or the absence of EGTA in order to obtain activated or aggregated platelets. Triton-insoluble residues of activated and aggregated platelets were electrophoresed on 4.5-15% linear gradient SDS polyacrylamide gel, visualized by Coomassie blue stain and scanned with a densitometer.

RESULTS

On the basis of the response to thrombin, group T1 was composed of subjects showing completely normal functional behaviour, while group T2 included patients who were more active than controls in response to low thrombin concentrations. By comparing the densitometric data of the two groups of patients it is possible to show that cytoskeleton structures in the T2 group have a significantly higher protein content than normals and than group T1. Platelet treatment with high thrombin concentrations cancels every difference between the two groups of patients with the exceptions of a 33-Kd protein and actin, which remain higher in group T2.

CONCLUSION

Abnormal platelet function and quantitative cytoskeleton protein content can be correlated in patients affected with E.T. In the platelets of those patients who display higher responsiveness to thrombin, either altered calcium homeostasis or changes in membrane viscosity leading to altered thrombin receptor expression, or both these events, could be present.

Generation of hydrogen peroxide in resting and activated platelets

The production of hydrogen peroxide was measured by following the oxidation of dichlorofluorescein (DCFH) entrapped into platelets. Resting platelets produced nanomolar quantities of DCF, which was proportional to the concentration of platelets and was steady during 1 h of incubation. A significant increase of basal DCF fluorescence was induced by stimuli namely thrombin, arachidonic acid, the Ca2+ ionophore A23187 and PMA. The effect of agonists has been also measured in the presence of 3-amino-1,2,4-triazole (AT) or N-ethylmaleimide (NEM), inhibitors of catalase and glutathione peroxidase, respectively. A further significant enhancement of DCF produced in stimulated platelets was detected only in the presence of NEM. A correlation was found between the increase in DCF and externally added hydrogen peroxide or the oxidizing species formed by xanthine oxidase plus acetaldehyde. The yield was not affected by superoxide dismutase and was higher in the presence of AT or NEM. A cooperative effect in the presence of both inhibitors was shown. Glutathione peroxidase plus glutathione diminished the level of DCF to basal levels.

Increased hydrogen peroxide formation in platelets of patients affected with essential thrombocythaemia (ET)

The basal platelet level of reactive oxygen species and their enhancement following stimulation by different agonists were determined in a selected group of patients with essential thrombocythaemia (ET). Activated platelets had lower levels of superoxide anion and higher intracellular concentrations of hydrogen peroxide than controls. Higher levels of lipid peroxidation induced by N-ethylmaleimide were also observed. Measurement of the most important enzymes generating and scavenging these reactive oxygen species revealed increased specific activities of NADPH-cytochrome c reductase and superoxide dismutase and a decrease in platelet catalase activity in patients with ET. Since an abnormal production of oxygen radicals seems to be implicated in various pathological conditions and aging processes, the increased amount of hydrogen peroxide found in platelets of patients could be involved in some of the platelet alterations described in ET.

Treatment of proteoglycan aggregates with physeal enzymes reduces their ability to inhibit hydroxyapatite proliferation in a gelatin gel

In vitro, cartilage proteoglycans (PGs) are effective inhibitors of hydroxyapatite formation and growth. Their inhibitory ability decreases with decreasing PG size and charge density. It has been suggested that the enzyme-mediated alteration in the size and conformation of PGs in the growth plate may similarly facilitate the calcification process. In this study, a gelatin gel system was used to monitor hydroxyapatite formation and growth in the presence of proteoglycan aggregates, before and after enzyme treatment. To reproduce the physeal degradation cascade, an enzyme preparation was used that contained all of the growth plate enzymes. At a concentration of 500 micrograms/ml, the untreated proteoglycan aggregates reduced the amount of mineral formed by 30%. When the aggregates were treated with the heat-inactivated enzyme, the same extent of inhibition was found. In contrast, treating the aggregates with the crude growth plate enzyme preparation removed all the inhibitory ability, such that 500 micrograms/ml of proteoglycan preparation yielded 10% more mineral than the controls. Treatment of the aggregates with chondroitinase ABC and trypsin, similarly removed all the inhibitory ability. These data, suggest that enzymatic degradation of proteoglycans may contribute to the regulation of growth plate calcification.

Hydroxyapatite formation in the presence of proteoglycans of reduced sulfate content: studies in the brachymorphic mouse

Proteoglycans from the brachymorphic (bm/bm) mouse have a reduced sulfate content due to the impaired activity of adenosine phosphosulfate phosphokinase in these animals. X-ray diffraction and infrared analyses of the mineral from the calcified cartilage of the bm/bm mice demonstrate the presence of significantly larger and more perfect hydroxyapatite crystals of lower carbonate to phosphate content than crystals found in the control animals. No differences were seen in the mineral content, crystallite size, CO3:PO4 ratio, or infrared splitting factors measured in the diaphyseal bone from these animals. Electron microscopic examination similarly shows larger, more disorganized crystals in the bm/bm animals' calcified cartilage as contrasted with controls. In vitro, proteoglycan aggregates from these dwarf mice are shown in a collagen gel-growth system to be less effective inhibitors of hydroxyapatite formation and growth than similarly size sulfated proteoglycans from age-matched control animals. The proteoglycans from the control mice were comparable in inhibitory ability to proteoglycan aggregates extracted from fetal bovine epiphyses. The in vitro and in vivo mineral parameters suggest the importance of sulfate for the interaction between proteoglycans and mineral in growth plate calcification.

Oxidative metabolism of human platelets

The reducing capacity toward cytochrome c present in human resting platelets increases upon platelet stimulation, and is partially inhibited by superoxide dismutase. This activity therefore represents the generation of superoxide anion. In order to evaluate hydrogen peroxide formation a quantitative assay by mean of dichlorofluorescin (DCFH) has been set up. The DCFH, trapped inside the cell, is oxidized by hydrogen peroxide to the fluorescent compound DCF. Basal DCF increases during activation of platelets by agonists. Arachidonic acid, calcium ionophore A23187 and to a lesser extent PMA and thrombin are the most effective. N-ethylmaleimide induces a dose-dependent DCFH oxidation and potentiates the effect of agonists. NAD(P)H--cytochrome c reductase enzyme, which catalyzes superoxide anion production, is present in platelets at high specific activity, as well as those enzymes who protect the cells from oxygen reactive species.

Mode of action of 2-(diethylamino)-7-ethoxychromone on human platelets

The in vitro effect of 2-(diethylamino)-7-ethoxychromone (RC39XVIII) on human platelet aggregation induced by several agonists and on thromboxane B2 formation, granule release and intracellular cAMP elevation has been studied. The chromosome-derivative exerts a dose-dependent inhibitory effect on aggregation produced by U46619, arachidonic acid, thrombin, collagen and ADP. RC39XVIII inhibits aggregation, TxB2 formation and granule release in parallel. Moreover the drug potentiates cAMP accumulation induced by iloprost and forskolin. The drug also inhibits soluble cAMP phosphodiesterase in a dose-dependent manner. No effect on adenylate cyclase activity measured in platelet membranes was evident.

Antiplatelet effect of 2-(diethylamino)-7-hydroxychromone

In this study the in vitro influence of 2-(diethylamino)-7-hydroxychromone (RC39II) on platelet aggregating responses, thromboxane A2 (TxA2) production, release reaction and intraplatelet cyclic AMP (cAMP) content has been investigated. The drug exerts a dose-dependent inhibitory effect on aggregating response to arachidonic acid, U46619, thrombin, collagen and calcium ionophore A23187. Inhibiting concentrations of RC39II also prevent platelet release reaction and TxA2 formation. RC39II potentiates platelet cAMP accumulation by Iloprost. Several studies, carried out on soluble cAMP phosphodiesterase (PDE) have shown that the drug inhibits phosphodiesterase in a dose-dependent manner. No effect was shown on adenylate cyclase activity from platelet membranes.

Concentration-dependent effects of dentin phosphophoryn in the regulation of in vitro hydroxyapatite formation and growth

The effect of dentin phosphophoryn on hydroxyapatite formation and growth was studied in an in vitro gelatin gel diffusion system. Phosphophoryn, in low concentrations (0.010-1 microgram/ml) promoted de novo hydroxyapatite formation; at a higher concentration (100 micrograms/ml) in the same system, the dentin matrix protein inhibited hydroxyapatite growth. Similar inhibition of hydroxyapatite growth was seen in solution. The intact phosphophoryn was not essential for either inhibition of seeded growth or promotion of mineralization, since the formic acid degraded protein was comparably effective. Transmission electron microscopy of the precipitates formed at 7 days showed no significant differences in crystallite size distribution in the presence and absence of phosphophoryn. However there was a dose-dependent decrease in the number of mineral clusters formed in the presence of increasing amounts of phosphophoryn, suggesting inhibition of secondary nucleation. These data provide support for the postulated 'multifunctional' role of the dentin phosphoprotein in the mineralization process.

Platelets of patients affected with essential thrombocythemia are abnormal in plasma membrane and adenine nucleotide content

The adenine nucleotides present in distinct cellular compartments of platelets of 27 patients affected with essential thrombocythemia have been measured. In order to quantify granule-bound nucleotides and adenylic cytoplasmic pool, platelets have been stimulated with thrombin or treated with increasing digitonin concentrations, respectively. Among patients, we have identified two groups: 12 patients (Group 1) had normal platelet level of ATP and ADP both in dense granules as well as in cytoplasmic pool. The other patients (Group 2) had granule ATP and ADP significantly lower and ATP/ADP ratio significantly higher than controls. In these patients an increase in hypoxanthine level, derived from metabolic ATP degradation occurring during stimulation, was observed. In addition, in the latter patients an increased resistance of plasma membrane to digitonin was shown, suggesting that membrane fluidity should be reduced owing to a modified cholesterol/phospholipid ratio.

Protein composition in resting and activated platelets of type I diabetic patients

Protein composition of platelets of eleven type I diabetic patients and thirteen control subjects were analyzed by SDS polyacrylamide gel electrophoresis. Bands have been scanned and quantified. No significant difference was shown between controls and patients in any of the bands identified in electrophoretic patterns of whole platelet, membrane fraction, resting, activated and aggregated cytoskeleton. Data suggest that alterations observed in platelet function of diabetic patients cannot be connected to the changes in protein composition.

Clinical aspects of visceral pain

Pain producing stimuli in viscera are different from the stimuli which induce somatic pain. Two main clinical aspects of visceral pain are observed: true visceral pain and referred pain. The distinction among visceral pain, referred pain and somatic pain may be difficult in the so-called "intricate conditions". Furthermore, mnemonic traces may play an important role in determining clinical aspects of visceral pain.

The effects of noncollagenous matrix proteins on hydroxyapatite formation and proliferation in a collagen gel system

The effects of several noncollagenous matrix proteins on hydroxyapatite formation and growth were studied in a dynamic collagen gel system. In this system growth plate proteoglycan aggregates at concentrations of 1-10 micrograms/ml were effective inhibitors, desulfated aggregates from brachymorphic mice were less effective. Phosphophoryn at 1-100 micrograms/ml had no effect on formation; 60-120 micrograms/ml retarded mineral growth. Type X collagen at concentrations of 50-300 micrograms/ml had no effect on formation or growth.

Inhibition of the glycolytic pathway by methylglyoxal in human platelets

The incubation of human platelets with methylglyoxal and glucose produces a rapid transformation of the ketoaldehyde to D-lactate by the glyoxalase system and a partial reduction in GSH. Glucose utilization is affected at the level of the glycolytic pathway. No effect of the ketoaldehyde on glycogenolysis and glucose oxidation through the hexose monophosphate shunt was demonstrated. Phosphofructokinase, fructose 1,6 diphosphate (F1, 6DP) aldolase, glyceraldehyde 3-phosphate dehydrogenase and 3-phosphoglycerate mutase were mostly inhibited by methylglyoxal. A decrease in lactate and pyruvate formation and an accumulation of some glycolytic intermediates (fructose 1,6 diphosphate, dihydroxyacetone phosphate, 3-phosphoglycerate) was observed. Moreover methylglyoxal induced a fall in the metabolic ATP concentration. Since methylglyoxal is an intermediate of the glycolytic bypass system from dihydroxyacetone phosphate to D-lactate, it may be assumed that ketoaldehyde exerts a regulating effect on triose metabolism.

Alkaline extraction and reverse-phase high-performance liquid chromatography of adenine and pyridine nucleotides in human platelets

The levels of adenine (ATP, ADP, AMP) and pyridine (NAD, NADH) nucleotides in human platelets have been measured by a simple and reproducible method. A rapid alkaline extraction allows a complete recovery of the compounds concerned. The metabolic ATP and ADP in the cytosolic fraction, the amount released upon thrombin stimulation, and the ADP bound to F-actin have also been evaluated. Analysis was performed by reverse-phase, isocratic high-performance liquid chromatography on a 5-microns Lichrosorb RP-18 column with uv detection at 254 nm.

Platelet glucose metabolism in type I diabetic subjects

In a group of 35 type I diabetics, platelet glycolytic and glycogenolytic flux have been measured. In type I diabetic platelets glucose uptake was significantly reduced. The quite normal lactate production was obtained by a faster utilization of stored glycogen in these patients. Specific activity of glycolytic enzymes was normal, in particular the levels and kinetic properties of soluble and membrane bound hexokinase. The rate of glucose flux through the hexose monophosphate shunt, measured either in resting or in arachidonic acid-stimulated platelets, was normal in diabetics. Kinetics of glucose transport across plasma membrane have also been determined. Km was significantly increased in diabetic patients. No changes was shown in Vmax. Modifications present in membrane organization of patients could involve glucose transport protein.